Hi, I am from the Netherlands and new on this forum. A Kef enthusiast, I have a Reference 101, and I had the 104aB and Cadenza in the past.
I have a few new old stock SA10054 units. Google does not give any search results on this. It is a spare part the oval but on the backside there is a coil. And also the connections for the cables are there. It looks like some part of the 'active' B139, only without the magnet. Who can tell me more about this particular spare part SA10054? For what purpose Kef made these?
Thanks!

If you are talking about item 712364902 then it looks like a normal B139 with magnet removed.
The other unit has the aluminium mass loading ring attached.
Note that one unit does not have the aluminium foil on the back of the driver.

I know that Sonus Faber (Extrema) once used a modified passive B139 with switchable resistors accross the coil to tune performance . The drone was on the back and a normal round unit at the front.

OK, well they are obviously B139 and BD139 service replacement assemblies then; B139 with voice coils and BD139 with the steel tube.

Because the B139 magnet assembly was screwed on via the square oversize top plate, repairs could be done by relatively unskilled agents, re-using the original magnet. VC centering was automatic, as the top plate semi-shear dowels located in holes on the chassis. KEF also saved on shipping costs to their agents.